Major trends in integrated circuit design include reduction in power supply voltages and transistor dimensions. The power supply voltage reduction is related to the transistor size reduction, as well as to other measures of circuit performance.
One measure of a transistor that has importance is the thickness of its gate oxide layer. On a basic level, a transistor includes two doped source and drain semiconductor regions spaced apart from each other in a differently doped semiconductor body region. A conducting gate is formed over the portion of the body separating the source and drain. The gate oxide layer separates the gate from the body portion over which it is formed. The transistor operates, at least in part, by applying a voltage to the gate, which excites a charge state in the body underneath the gate to form a conduction channel.
The gate oxide thickness correlates to performance and operational measures of the transistor. A thinner gate oxide produces a faster device, having a higher bandwidth, or unity-gain frequency fT. A thicker gate oxide produces a slower device, having a lower unity-gain frequency fT. However, a thinner gate oxide also reduces the maximum voltage to which the transistor can be exposed without being damaged, and thus the maximum voltage swing that the transistor can supply in a circuit. A thicker gate oxide, by contrast, can withstand a higher voltage without being damaged, and thus can provide a greater voltage swing. In an exemplary 65 nm minimum-feature-size integrated circuit process, successively greater gate-oxide thicknesses result in transistors capable of operating under maximum power supply voltages (and thus providing correspondingly related output-voltage swings) of 1.0V, 1.8V, 2.5V, and 3.3V, respectively. However, the thickest oxide, capable of providing the greatest output-voltage swing, has a unity-gain frequency fT that is only one-tenth as great as that of the thinnest-oxide transistor, which has the least output-voltage-swing capability.
The tradeoff between speed and output-voltage swing, as embodied as a result of the performance implications of varying gate-oxide thicknesses, surfaces as a corresponding tradeoff in the design of circuits using transistors. For example, use of thin-oxide transistors in an amplifier results in the amplifier having a high bandwidth, but being capable of only a limited output-voltage swing. By contrast, use of thick-oxide transistors in an amplifier results in the amplifier being capable of a higher output-voltage swing due to its ability to operate under a greater power supply voltage, but also having has a lower bandwidth.
Thus, a need exists to overcome the limitations on amplifier design, and circuit design in general, imposed by this tradeoff between bandwidth and output-voltage swing resulting from transistor properties correlating to gate-oxide thickness.